I still do not know why they do not have a seat tube. The seat tube should help aero dynamics by creating a much better leading edge for the wheel than a tire. Even the part that does not shield the tire would create a bunch of lift of low yaw numbers, which is helpful. It made sense when the beam had some suspension in it, but both the Dimond and the Falco are rigid.
I think they are making a comeback because they look really cool. The dimond is also pretty aero, but I think it would be even more aero with a seat tube.
I tend to read beam bikes in much the same way--and I used to race a Softride back when they were in production. Today's top frames are highly optimized, and it's difficult to beat a triangular structure for strength and stability. There's no free lunch--taking material away from one area generally means that it needs to be used somewhere else. All frame designs represent some compromise, and I'm not completely sold on the notion that beam bikes in general
automatically will be faster than a frame with a seat tube.